1. Field of the Invention
The present invention relates generally to a method for improving the recovery of cesium-131 (Cs-131) from barium (Ba). Uses of the Cs-131 purified by the method include cancer research and treatment, such as for use in brachytherapy implant seeds independent of method of fabrication.
2. Description of the Related Art
Radiation therapy (radiotherapy) refers to the treatment of diseases, including primarily the treatment of tumors such as cancer, with radiation. Radiotherapy is used to destroy malignant or unwanted tissue without causing excessive damage to the nearby healthy tissues.
Ionizing radiation can be used to selectively destroy cancerous cells contained within healthy tissue. Malignant cells are normally more sensitive to radiation than healthy cells. Therefore, by applying radiation of the correct amount over the ideal time period, it is possible to destroy essentially all of the undesired cancer cells while saving or minimizing damage to the healthy tissue. For many decades, localized cancer has often been cured by the application of a carefully determined quantity of ionizing radiation during an appropriate period of time. Various methods have been developed for irradiating cancerous tissue while minimizing damage to the nearby healthy tissue. Such methods include the use of high-energy radiation beams from linear accelerators and other devices designed for use in external beam radiotherapy.
Another method of radiotherapy comprises brachytherapy. Here, radioactive substances in the form of seeds, needles, wires or catheters are implanted permanently or temporarily directed into/near the cancerous tumor. Historically, radioactive materials used have included radon, radium and iridium-192. More recently, the radioactive isotopes Cs-131, iodine-125 (I-215), and palladium-103 (Pd-103) have been used. Examples are described in U.S. Pat. Nos. 3,351,049; 4,323,055; and 4,784,116.
During the last 30 years, numerous articles have been published on the use of I-125 and Pd-103 in treating prostate cancer. Despite the demonstrated success in certain regards of I-125 and Pd-103, there are certain disadvantages and limitations in their use. While the total dose can be controlled by the quantity and spacing of the seeds, the dose rate is set by the half-life of the radioisotope (60 days for I-125 and 17 days for Pd-103). For use in faster growing tumors, the radiation should be delivered to the cancerous cells at a faster rate, while simultaneously preserving all of the advantages of using a soft x-ray emitting radioisotope. Such cancers are often found in the brain, lung, pancreas, prostate and other tissues.
Cesium-131 (Cs-131) is a radionuclide product that is ideally suited for use in brachytherapy (cancer treatment using interstitial implants, i.e., “radioactive seeds”). The short half-life of Cs-131 makes the seeds effective against faster growing tumors such as those found in the brain, lung, and other sites. While prostate cancer is generally considered slower growing, certain prostate cancers are more aggressive and more appropriately treated using an isotope with a shorter half-life such as Cs-131. The shorter half-life of Cs-131 is equally effective against the slower growing tumors and thus is applicable for treatment where the aggressiveness of the tumor is not well known in advance (C. I. Armpilia et al., Int. J. Radiat. Oncol. Biol. Phys. 55:378-385 (2003)).
Cesium-131 is produced by radioactive decay from neutron irradiated naturally occurring Ba-130 (natural Ba comprises about 0.1% Ba-130) or from enriched barium containing additional Ba-130, which captures a neutron, becoming barium-131 (Ba-131). The source of the neutrons can be a nuclear reactor or other neutron generating devices (e.g., neutron generators). Barium-131 then decays with an 11.7-day half-life to cesium-131, which subsequently decays with a 9.7-day half-life to stable xenon-130. Thus, with the decay of Ba-131 comes the buildup of Cs-131. To separate the Cs-131, the barium target is “milked” multiple times over selected intervals such as 7 to 14 days, as Ba-131 decays to Cs-131. With each “milking,” the Curies of Cs-131 present and the gram ratio of Cs to total Ba decreases (less Cs-131 per gram of Ba) until it is not economically of value to continue to “milk the cow” (e.g., after approximately 40 days). The barium “target” can then be returned to the reactor for further irradiation (if sufficient Ba-130 is present) or discarded.
In order for the Cs-131 product to be useful, the Cs-131 must be exceptionally pure, free from other metal (e.g., barium, calcium, iron, cobalt, etc.) and radioactive ions including Ba-131. A typical radionuclide purity acceptance criteria for Cs-131 is >99.9% Cs-131 and <0.01% Ba-131.
The objective in producing highly purified Cs-131 from irradiated barium is to completely separate less than 7×10−7 grams (0.7 μg) of Cs from each gram (1,000,000 μg) of barium “target.” A typical target size may range from several grams to several kilograms of Ba, depending on whether enriched Ba-130 or natural target is used in irradiation (natural Ba comprises about 0.1% Ba-130). Typically, irradiated Ba targets comprise various Ba salts. Most often barium carbonate is used. Because Cs-131 is formed in the BaCO3 crystal structure during decay of Ba-131, it is assumed that the Ba “target” must first be dissolved to release the very soluble Cs ion.
As noted above, Cs is a very small fraction (about less than 0.0001%) of the irradiated barium target, and thus it is beneficial to be able to recover the Cs in good yield. This is particularly true where processes for production of Cs-131 from Ba are scaled-up. Current approaches typically involve dissolution of the Ba targets in acid to release Cs+1 ions. Commonly acetic acid is used for dissolution. The dissolution step is followed by precipitation of Ba in the form of a compound with limited solubility in water, while Cs+1 ions remain in solution and thus separated from Ba. Commonly, Ba is precipitated as carbonate using ammonium carbonate (NH4)2CO3 solution as the precipitating reagent. While other carbonates can be used (e.g., Li, Na, etc.), the advantage of using precipitating reagents based on ammonium salts is the ease of separating Cs from ammonium ions.
Following precipitation, the liquid containing Cs is separated from the barium precipitate by common methods (such as filtering or centrifuging) followed by evaporation to dryness of the acetate or other organic acid salts formed during Cs-131 separation from Ba carbonate. This is then followed by use of dilute acetic acid for dissolution of Cs salts. A disadvantage is that currently the recoveries using such procedures are generally on the order of only 30%-50%. The remaining balance of Cs-131 is associated with an organic, carbonaceous residue formed during evaporation of the filtrate solution containing Cs, ammonium acetate and ammonium carbonate salts. One further disadvantage of the current approaches for using ammonium carbonate solution as a precipitant is the limited solubility of the ammonium carbonate reagent in water (less than 3 moles/L). Limited solubility of the precipitating reagent results in an undesirable increase in the total volume of solution remaining after the Ba precipitation step. Increased solution volume requires larger scale equipment and lengthens the evaporation process. These are particularly problematic for implementation of large scale (>100 g) target processing. In this manner, the disadvantages to the current approach of using ammonium carbonate as the precipitating reagent are associated with the formation of a carbonaceous residue during the evaporation of ammonium acetate and the limited solubility of this reagent in water.
Due to the need for better Cs-131 recoveries and the deficiencies in the current approaches in the art, there is a need for improved methods. The present invention fulfills this need and further provides other related advantages.